Magnetic degaussing systems of various kinds are known in the art. Typically, magnetic fields of varying strength and direction are applied to the item to be degaussed forcing the magnetization within the object to change thereby destroying any patterns therein. Magnetic degaussing systems have become increasingly important with the increasing use of magnetic data storage. Data stored magnetically can remain on the storage medium for long periods of time after its use. For example, a computer disk's data can be retrieved even after a user has “erased” the data from the disk because the old data will not be changed until new data is written over that segment of the disk. If another person were to obtain the disk, that person may be able to access information from that disk.
In the art of bulk degaussing of magnetic data storage media, electrically powered degaussing systems are commonly used. For example, laminated steel cores of extruded “U” shapes in association with electrical windings are generally recognized as one configuration suitable for erasure of magnetic data storage media. Similarly, “E” shaped cores may be used. Pairs of such cores are often configured opposite each other with like poles facing, although single sided and offset configurations are also known in the art. Although such configurations are suitable for some situations, these systems have the disadvantage of needing a power source to create the fields necessary for magnetic data storage media erasure.
More recently, the discovery and improvement of rare earth permanent magnets have made the generation of magnetic fields of strengths suitable for bulk media erasure using permanent magnets practical. Such permanent magnets can be arranged with steel elements into magnetic circuits that act much like their electric counterparts. The weight requirements of permanent magnet systems are about equal to the electric systems. Further, the zero power input required by permanent magnets offsets higher production costs as compared to electric systems.
Another advantage of permanent magnet systems includes the use of individual elements, which may be off-the-shelf items, rather than trying to fabricate large elements or permanently magnetizing a single large shape. For example, it is known that a total of eight 2-inch by 2-inch by 1-inch neodymium-iron-boron (NeFeB) blocks, magnetized in the 1-inch direction, can be adhered by magnetic attraction onto steel plates as groups of four blocks thereby forming two 2-inch by 8-inch poles, a classic “U” shape magnet of 8-inch depth. Two such “U” shapes can be configured with like poles facing in repulsion across a gap suited to passage of 1-inch thick magnetic media. Such an assemblage can apply a magnetic field with good uniformity and at least 6000 gauss to every point in a common form factor for magnetic data storage media passing through that field. It is understood that at least a second passage of a magnetic storage medium through the field with a different orientation between the storage medium and the magnetic field is necessary to impart the desired change within the storage medium to affect magnetic data storage erasure.
Despite the advantages of these known permanent magnet systems, certain drawbacks exist. For instance, magnetic data storage media are being developed with increasing magnetic coercivities such that much stronger fields must be applied to completely erase the media. As such, the 6000 gauss strength achieved by known permanent magnet bulk degaussing systems is marginal with respect to the emerging media's coercivities.
Attempts to increase the strength of the known permanent magnet bulk degaussing systems by scaling up the systems, however, quickly lead to diminishing returns. Such scaling of prior art includes stacking off-the-shelf elements in their direction of magnetization, placing elements side by side on the steel plates, stacking and placing elements, or substituting larger custom-made elements or magnets for the off-the-shelf elements. It is generally recognized in the art of bulk degaussing that worst case field strength drives performance and that a measure of nonuniformity in field strength can be tolerated. It is also known that attempts to furnish field strengths sufficient for erasure of magnetic storage media with higher coercivities using various prior art facing “U” arrangements would require at least a correspondingly increased amount of NeFeB or other magnetic material plus thick steel components needed to complete the required magnetic circuit. Such a system would result in an unacceptable degree of field strength nonuniformity across the gap. In particular, the diminishing returns from prior art scaling using NeFeB elements arise due to flux leakage from NeFeB elements to each other and into the steel plates where media cannot be placed to affect erasure.
Additionally, any such scaling results in larger volume, increased weight, and greater cost. It is well known that in the assembly of the prior art permanent magnet systems, regions of both magnetic attraction and magnetic repulsion will arise between various elements and members. For example, magnets are attracted to steel plates and to each other when stacked with unlike poles facing. Conversely, placing magnets adjacent to each other with the same magnetic direction causes repulsion, as does placing like poles facing each other across a gap. To counter such forces, framework members must be added. In the prior devices, a thick steel plate serves a dual role as a required component of the magnetic circuit and as one of the framework members, but other members generally must be of nonmagnetic materials to avoid undesirable magnetic circuit paths or unnecessary magnetic field fringing effects. In particular, prior devices require an attraction-countering member between unlike poles, which experiences extreme compressive force, and this member cannot be magnetic steel. These structural requirements only become aggravated with the scaling of the prior permanent magnet devices.
Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions and/or relative positioning of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of various embodiments of the present invention. Also, common but well-understood elements that are useful or necessary in a commercially feasible embodiment are often not depicted in order to facilitate a less obstructed view of these various embodiments of the present invention. It will also be understood that the terms and expressions used herein have the ordinary meaning as is accorded to such terms and expressions with respect to their corresponding respective areas of inquiry and study except where specific meanings have otherwise been set forth herein.